GALILEO INFRARED OBSERVATIONS OF THE SHOEMAKER-LEVY-9 G-IMPACT FIREBALL - A PRELIMINARY-REPORT

The Galileo spacecraft was fortuitously situated for a direct view of the impacts of the fragments of comet Shoemaker-Levy 9 in Jupiter's at mosphere. The Galileo Near Infrared Mapping Spectrometer (NIMS) instru ment observed several of the impact events in several discrete bands a nd with a temporal resolution of roughly five seconds. Data have been received for the G impact showing two phases of strong infrared emissi on. The first phase is approximately one minute in duration and corres ponds to the initial fireball and early plume development. This is fol lowed six minutes later by the onset of heating by plume ejecta fallin g back on the upper atmosphere. This report provides a preliminary des cription of the fireball phase. The first detection of the G fireball occurred at 07:33:37 UT on July 18, 1994, approximately five seconds a fter the initial signal recorded by the Galileo Photopolarimeter-Radio meter (PPR) instrument. The preceding NIMS measurement, occurring appr oximately one second before the initial PPR signal, showed no evidence of fireball emission. The detected duration of the fireball at 4.38 m u m was 70 seconds. Spectra in the first half of this period show blac kbody-like emission, with absorption features from overlying methane a nd molecular hydrogen. The strengths of these features place the fireb all in the upper troposphere and lower stratosphere, above the ammonia cloud layer. The emitting surface rises and accelerates, achieving a velocity of 2 - 3 km/sec after 25 seconds, in qualitative agreement wi th that expected for an explosion in an inhomogeneous atmosphere. The Galileo spacecraft, en route to Jupiter, was in a position to obtain a direct view of the impacts of Comet Shoemaker-Levy 9 fragments on the nightside of Jupiter, providing an opportunity to investigate the ear ly temporal evolution of the impact events. It was predicted that the comet fragments would produce high temperature bolides as they entered the atmosphere and then explode, producing hot fireballs which would rise, expand, and cool (Sekanina, 1993; Zahnle and Mac Low, 1994; Chev alier and Sarazin, 1994; Ahrens et al., 1994; Boslough et al., 1994). Much of the predicted radiation occurs in the infrared region, and tim e-resolved infrared spectral observations, obtained over a broad wavel ength range, are ideal for studying these phenomena.